I would say education. It's a proven fact that knowledge increases a person's IQ and which is why people with higher educations are more ethical. But sure, all that inbreed/incest fucking can't be very good for Theist genetics! -with love from an Agnostic
I count three generalizations there: IQ is an unreliable indicator of an individual's intelligence; as evidence, I present our current President, who is--by several accounts--the smartest person to hold office, if not the most brilliant man to stride the North American continent. Also Henry Halleck, who was by all accounts the most brilliant general of the Union army--and also among its most useless. I should also point out that a person's intellect doesn't necessarily have as much bearing on their religious status as you may think: Woodrow Wilson was deeply religious and was a professor and then President of Princeton, one of the country's top universities. He was also an inveterate racist and rather naive about international politics, but I don't see how either of those may have been reduced by agnosticism or outright atheism. As to whether people with higher educations are more ethical, try googling "The Smartest People in the Room"; see where that gets you. The last point is gratuitously and vulgarly absurd so I'll treat it as the joke it obviously is, shake my head, and think more about the other two points.
Smart doesn't make you street smart or makes you the best in everything. I've a higher IQ score than average and I don't feel all that smart, and I suffer from Dyslexia which makes me rather shit at spelling, and yet I spell better in multiple languages than other people, who can hardly even do with, despite not having Dyslexia. Maybe that is where my IQ kicks in? Who knows. IQ is usually measured in logic, so that doesn't mean that just because I might be more logical than most people won't mean that I don't really understand how people think, because people aren't very logical... which is one of their greatest weaknesses.
Darwin was a smart guy who was religious, but after his research he lost faith in God and became an Atheist. So those people you're referring to, who believe that snakes can talk; are what we call "stupid smart people". People who really shouldn't believe in stuff like that, and yet they do. Beside, it's shown that people that are higher educated are less likely to be religious. Which is why 90+% of Scientists are Atheists, and have higher IQs than that of religious people. So please stop saying these silly things, that has little to no hold in reality.
It was no joke, just me mocking the Bible. You should read it sometime; lots of incest fucking going on there. And any educated person knows that inbreeding isn't exactly a "good" thing, when you consider it on a genetic level. If it's moral or not? I don't give a fuck. It's not my place to judge a brother loving his sister, or whatever the case might be.
Actually, Darwin considered himself an agnostic. Beware: there are more "stupid smart people" than you think. You may find them in your heroes' houses. I've read it; I thought you were making a crack about Southerners.